This disclosure pertains to charged-particle-beam (CPB) optical systems and to systems, such as CPB microlithography systems, incorporating such optical systems. Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, magnetic-recording heads, displays, and micromachines. More specifically, the disclosure pertains to deflectors used in CPB optical systems and to methods for manufacturing such deflectors. Compared to conventional deflectors, the deflectors disclosed herein exhibit reduced aberration, produce strong beam-deflecting fields at low excitation currents being applied to the deflectors, exhibit low fluctuation of their deflecting magnetic fields with changes in temperature, and are relatively little influenced by eddy currents.
As the limitations of optical microlithography have become more apparent, a large research and development effort in recent years has been directed to charged-particle-beam (CPB) microlithography as a primary candidate for xe2x80x9cnext generationxe2x80x9d lithography technology. By using a charged particle beam (e.g., an electron beam), CPB microlithography offers prospects of improved pattern-transfer resolution, compared to optical microlithography, for reasons similar to the reasons for which electron microscopy produces better imaging resolution than optical microscopy. Thus, CPB microlithography offers the prospect of producing microelectronic devices (e.g., semiconductor integrated circuits) having smaller and more densely packed active-circuit elements than can be produced by conventional means. In CPB microlithography, exemplified by electron-beam projection microlithography, a pattern is defined on an xe2x80x9cEB reticle,xe2x80x9d from which the pattern is xe2x80x9ctransferredxe2x80x9d with reduction (demagnification) to a xe2x80x9csensitivexe2x80x9d substrate using a projection-optical system of a CPB optical system. xe2x80x9cSensitivexe2x80x9d means that the surface of the substrate is coated with a substance, termed a xe2x80x9cresist,xe2x80x9d that is imprintable with an image of the pattern as carried to the surface by the beam.
A reticle suitable for use in CPB microlithography typically is fabricated from a silicon wafer having a diameter of, for example, about 200 mm. The exposure field of an electron-optical system is only about 250 xcexcm wide, rendering most full-die exposures from a reticle currently impossible. Consequently, exposure of an entire pattern from the reticle to the substrate involves defining the pattern on a reticle that is xe2x80x9csegmentedxe2x80x9d into a large number of portions (usually termed xe2x80x9csubfieldsxe2x80x9d) each defining a respective portion of the overall pattern. The subfields are exposed in respective exposure xe2x80x9cshotsxe2x80x9d to the substrate, on which the subfield images are placed to form a contiguous pattern on the substrate. Thus, large dies can be exposed, including dies having dimensions of tens of millimeters square on the substrate. An exemplary technique in this regard is disclosed in U.S. Pat. No. 4,376,249, incorporated herein by reference.
With any of various types of CPB microlithography systems, an ability to deflect the charged particle beam laterally with respect to the optical axis is absolutely essential for having a functional system. Consequently, to such end, all CPB microlithography systems comprise multiple deflectors.
One type of deflector frequently employed in electron-beam optical systems is a xe2x80x9csaddlexe2x80x9d deflector. A saddle deflector is produced by winding an electrical coil around a square bobbin, then bending the coil part way in a saddle manner around the outer surface of a cylinder. Unfortunately, this method of forming a saddle coil yields inaccurately configured coil windings and poor precision from one deflector to the next. Manufacturing difficulties also are encountered while positioning deflector cores relative to the coil windings.
Another type of deflector frequently used in electron-beam optical systems is denoted a xe2x80x9cvane-yokexe2x80x9d type of toroidal deflector, as shown in FIGS. 16(a)-16(b). A coil 34 of such a deflector is made by cutting (e.g., wire-cutting) a copper sheet to form a planar coil. A separate coil 34 is positioned on and applied to each side of a rigid, planar, insulative substrate 33 (e.g., quartz). Thus, each substrate 33 is provided with a xe2x80x9cclockwisexe2x80x9d coil 34 and a xe2x80x9ccounter-clockwisexe2x80x9d coil 34. The clockwise coil 34 is applied to one side of the substrate 33, and the counter-clockwise coil is applied to the opposite side of the substrate. The respective inner termini of the coils 34 are electrically connected together, and the respective outer termini are connected to a power supply. Each such planar assembly is a respective xe2x80x9cvane.xe2x80x9d The vanes 32 are radially positioned relative to each other about an optical axis 36 to form the deflector 31.
In an electron beam, the constituent propagating electrons repel each other. Consequently, an image carried by and formed by the beam can exhibit distortion and/or blur, especially at higher beam currents. This phenomenon is commonly known as the xe2x80x9cCoulomb effect.xe2x80x9d If the beam current is reduced in an effort to decrease the Coulomb effect, then exposure time of an electron-beam microlithography system is lengthened, which can reduce the throughput of the system. xe2x80x9cThroughputxe2x80x9d is the number of workpieces (e.g., wafers) that can be processed (e.g., lithographically exposed) by the system per unit time.
Another way in which the Coulomb effect can be reduced is by decreasing the length of the column containing the electron-beam optical system. In a shorter column, the distance of beam propagation is correspondingly reduced, which reduces the time during which the electrons of the propagating beam are near each other sufficiently to repel each other. However, a shorter beam column usually results in the beam being deflected, by a given deflector, a shorter distance from the optical axis than experienced in a longer column. Hence, in a shorter column, achieving a desired lateral deflection of the beam requires that the deflector coil be energized with a higher electrical current than an otherwise similar deflector in a longer column. The elevated electrical current results in more heat being generated in the coil. Unless this heat is rapidly and efficiently dissipated from the deflector coil, the deflector itself is heated. Thus, the deflector exhibits a greater variation in temperature, which produces a correspondingly greater variation in performance.
Reducing the column length of a CPB optical system also requires that each deflector be made smaller than would be allowable in a longer column. As a result, the deflectors in a short column are very close to other components of the column, thereby concentrating heat in a smaller area around the deflector. Achieving sufficient cooling of the deflector for more accurate and precise operation is correspondingly more difficult. As a result, the deflectors tend to experience greater temperature fluctuation during operation, yielding correspondingly greater thermal expansion and contraction of the deflectors. As a deflector expands, the magnetic field generated by the deflector increases in magnitude, which increases the magnitude of beam deflection, at an applied current, imparted by the deflector. As the imaging position of the beam fluctuates with temperature changes of the deflector, the accuracy and precision with which the subfield images are stitched together on the substrate correspondingly fluctuates.
One way in which to increase the magnitude of the deflection field produced by a deflector energized with a relatively small electrical current is to configure the coil as being wound around a magnetic xe2x80x9ccore.xe2x80x9d Because deflectors usually generate high-frequency magnetic fields, ferrite often is used for the core because of its high electrical resistance, which is important for reducing eddy currents in the deflector. This type of deflector is able to create a relatively strong deflecting field in response to a relatively low current applied to the coil, and is therefore utilized in many different CPB optical devices.
Using an xe2x80x9cMOL lensxe2x80x9d (Moving Objective Lens; see Goto et al., Optik 48:255-270 (1977)) is a well-known technique for deflecting an electron beam in electron-beam microlithography systems. A MOL lens achieves deflection of an electron beam while reducing distortion and blur (aberration). A MOL lens conventionally comprises (a) a magnetic lens for converging the beam and (b) a deflector, comprising a ferrite core, for deflecting the beam. The lens and deflector are disposed concentrically around the optical axis of the MOL lens. Using such a lens configuration, it is possible to superpose on the lens field a second magnetic field having a direction that intersects the lens field. The resultant field imparts a lateral shift to the principal point of the lens. An electron beam passing through the lens in a generally axial direction is caused always to pass near the principal point of the lens, even whenever the principal point is laterally shifted. This movement of the principal point reduces deflection aberrations of the lens. Otherwise, merely deflecting an electron beam causes the beam to pass relatively far away from the principal point, which causes a correspondingly greater deflection aberration.
A MOL lens for a CPB optical system typically produces a respective lensing magnetic field that is substantially continuous. The deflector serving to shift the principal point of the MOL lens, in contrast, typically produces a beam-shifting field that rapidly changes in a high-frequency manner synchronously with deflection of the beam occurring outside the MOL lens. The high-frequency AC magnetic field produced by the deflector acts on the magnetic lens of the MOL lens, usually situated radially outwardly of the deflector. The magnetic lens typically comprises a copper-coil winding and a pole piece located peripherally relative to the coil winding. Whenever an AC magnetic field acts on such a magnetic lens, a corresponding eddy current is generated in the lens. The eddy current tends to slow the rate at which the deflecting field produced by the deflector can settle after being changed, which produces a corresponding reduction in the response rate of the deflector.
A technique as shown in FIGS. 17(a)-17(b) conventionally is used for preventing these eddy currents and/or their effects. FIG. 17(a) illustrates a conventional magnetic lens disposed coaxially with a deflector used to deflect an electron beam propagating in a generally axial direction through the lens. A deflecting coil 11 is disposed inboard, and a magnetic-lens coil 12 is disposed outboard relative to the axis 15. A pole piece 13 is disposed around the outer periphery of the magnetic-lens coil 12 so as to xe2x80x9ccoverxe2x80x9d the magnetic-lens coil 12. A ferrite xe2x80x9cstackxe2x80x9d 14 is situated concentrically between the deflecting coil 11 and the magnetic lens coil 12, and serves to magnetically shield the magnetic lens coil 12 from the AC magnetic field induced by the deflecting coil 11.
The effect of the ferrite stack 14 is shown in FIG. 17(b), which is a transverse section perpendicular to the optical axis 15 in FIG. 17(a). The magnetic field (denoted by lines of force 16) generated from the portion of the deflecting coil 11 located near the optical axis 15 passes near the optical axis 15 and then returns to the deflecting coil 11. Meanwhile, the magnetic field (denoted by lines of force 17) generated from the portion of the deflecting coil 11 located more remotely from the optical axis 15 extends into the ferrite stack 14 surrounding the deflecting coil 11, around the outside of the deflecting coil 11, and then back to the deflecting coil 11. The magnetic field extending around the outside of the deflecting coil 11 enters the surrounding ferrite stack 14 and does not reach the magnetic lens coil 12 or the pole piece 13.
A deflector used in a CPB microlithography system desirably comprises a coil and a core made with extremely high accuracy and precision. Otherwise, a difference will be exhibited between the expected magnetic field and the actual magnetic field produced by the deflector whenever the charged particle beam is being deflected by the deflector. This difference results in deflection aberrations, which can be a serious problem. Unfortunately, a sufficiently accurate method for manufacturing such a deflector has not yet been devised. Consequently, troublesome deflection aberrations always are encountered in CPB microlithography.
Also, the use of ferrite in a deflector poses two major problems. The first is that the magnetic permeability of ferrite varies with changes in temperature. In general, the Curie point of a material is the temperature at which the magnetic permeability of the material is at a minimum; the Curie point of ferrite is approximately 200xc2x0 C. Whenever such a deflector is used at room temperature, which is considerably below the Curie point of ferrite, the permeability of the deflector varies with deflector temperature.
The second problem is that considerable variation exists in the composition of ferrite. Ferrite is manufactured by mixing the constituent materials in a powdered state; mixing of powders is inherently not uniform. Also, magnetic characteristics of ferrite are affected by the conditions under which sintering occurs, which is another source of variation in the behavior of ferrite. In any event, the resulting variations in the permeability of ferrite result in undesirable astigmatism and the like in the deflector.
Therefore, in a conventional deflector having a ferrite core, a change in ferrite temperature causing a corresponding change in the permeability of the ferrite (due to the first problem noted above) causes a corresponding variation in the magnitude of the deflecting magnetic field produced by the deflector. As a result, whenever the deflector is energized, the charged particle beam is not deflected by the deflector in quite the desired manner. Also, due to the second problem noted above, astigmatism is greater.
The two ferrite problems noted above can cause the following to occur even in a MOL comprising a deflector including a ferrite stack. First, as noted above, magnetic permeability varies with corresponding changes in ferrite temperature. As shown in FIG. 17(b), part of the magnetic field from the deflector passes through to the inside of the ferrite stack. If the permeability of the ferrite were increased in this location, for example, the flux density (indicated by the number of lines of force) entering the ferrite stack also would increase. As a result, some of the magnetic flux that has crossed the optical axis passes to the outside of the deflector, causing a reduction of the deflection magnetic field on the optical axis. Thus, the magnitude of beam deflection produced by the deflector is not as expected. As noted above, the second problem causes increased astigmatic behavior and the like imparted to the beam.
In view of the shortcomings of the prior art as summarized above, the present invention provides, inter alia, deflectors exhibiting, compared to conventional deflectors, very low aberration while generating strong deflecting magnetic fields at low applied electrical current to coils of the deflectors. The low aberration is attributable to the magnetic-tape laminate used to form the core of the deflectors. The laminate of magnetic tape is more uniform than the ferrite conventionally used in deflector coils, and thus exhibits less variation in magnetic permeability. The deflectors also exhibit very little fluctuation in the magnitude of the deflecting magnetic field with variations in deflector temperature. Additionally, the deflectors are affected very little by eddy currents. Thus, the deflectors achieve high-speed positioning of the charged particle beam, which is especially suitable for high-precision CPB microlithography. Also provided are methods for manufacturing such deflectors.
According to a first aspect of the invention, toroidal deflectors are provided. One embodiment of such a deflector comprises an annular core including a laminate of magnetic tape. At least one coil is situated relative to the core; when the coil is electrically energized the deflector produces a deflecting magnetic field. In deflectors having multiple coils, the coils may be positioned at substantially equi-angular positions from one another relative to the core. The magnetic tape may comprise a foil of magnetic metal. The magnetic tape of the annular core can be configured so as to be electrically energized to generate a lens field superposed on the deflecting magnetic field.
Another embodiment of a toroidal deflector comprises multiple vanes radially positioned around an optical axis. Each vane comprises a planar substrate having a respective electrically energizable coil attached to at least one major surface of the vane. The coil forms a substantially spiral pattern and defines a coil interior. A core comprising a laminate of magnetic tape is positioned in the coil interior. The magnetic tape can be a foil of magnetic metal.
In the embodiment summarized above, a second coil can be affixed to the opposite side of the vane. The second coil forms a substantially spiral pattern that desirably mirrors the first spiral pattern. The second coil may similarly define a second coil interior, in which a second core may be positioned.
The core can be positioned in the coil interior by alignment with multiple positioning features defined in the coil interior. The positioning features desirably are electrically insulated from the core.
The core can be configured to have a thickness that varies with radial distance from the optical axis of the deflector. For example, the thickness of the core can increase with increasing radial distance from the optical axis.
The core can be divided into multiple core segments that are electrically insulated from one another.
According to another aspect of the invention, saddle deflectors are provided that are formed on a cylindrical substrate. In one embodiment an outer electrically energizable coil is affixed to the outer surface in a first spiral pattern. The outer coil also defines an outer coil interior. A core comprising a laminate of magnetic tape is affixed to the outer surface and positioned in the outer coil interior. The saddle deflector can include an inner electrically energizable coil affixed to the inner surface in a second spiral pattern that mirrors the first spiral pattern. The cylindrical substrate can further comprise at least one through-hole defining an aperture between the inner and the outer surfaces. Thus, an end of the outer coil and an end of the inner coil can be electrically connected to each other at one of the through-holes. The other ends of the outer and inner coils can be disposed substantially adjacent to one another on the outer surface by positioning the end of the inner coil through a second through-hole. These ends are connectable to a power supply with wires that desirably are twisted together.
The core can be divided into multiple core segments that are electrically insulated from one another.
According to yet another aspect of the invention, biaxial saddle deflectors are provided. An embodiment of such a biaxial saddle deflector comprises a first saddle deflector substantially similar to the saddle deflector summarized above. The biaxial saddle deflector further comprises a second saddle deflector substantially similar to the first saddle deflector except that the second saddle deflector has an outer diameter smaller than the inside diameter of the first saddle deflector. Thus, the second saddle deflector is insertable coaxially into the first saddle deflector. The second saddle deflector can be oriented axially ninety degrees from the first saddle deflector. The saddle deflectors can comprise respective flanges for positioning the second saddle deflector relative to the first saddle deflector.
The first and second saddle deflectors can include respective cores affixed to the respective outer surfaces of the deflectors and positioned in respective coil interiors. The cores desirably each comprise a laminate of magnetic tape. The cores can be divided in multiple core segments.
According to yet another aspect of the invention, methods are provided for manufacturing a vane for use in a vane-yoke deflector. In an embodiment of the method, a coil pattern of metal is formed on a surface of a planar substrate. The surface of the substrate is coated with a thick-film resist. A desired coil profile and a desired core profile are patterned onto the resist using lithography. Non-cured portions of the resist are removed to reveal the coil pattern and a respective core location. Conductive metal is deposited in the coil pattern to form the coil. A core comprising a laminate of magnetic tape can be placed at the core location using an adhesive or the like. The core can be positioned with the aid of multiple positioning features formed on the surface in the core location. The core can be divided into multiple portions and/or machined to a desired shape. For instance, the core may be machined so that its cross-section increases proportionately with increasing axial distance from the optical axis of the deflector.
According to yet another aspect of the invention, methods are provided for manufacturing saddle deflectors. In an embodiment of such a method, a metal film is deposited on an outer surface of a cylindrical substrate to form a metal layer. The substrate is progressively submerged endwise into photolithographic resin. A coil profile is patterned in the resin on the outer surface of the substrate using a laser beam. Non-patterned portions of the resin are removed to reveal a coil pattern. A conductive metal is deposited in the coil pattern to form a coil. Remaining resin is then removed. A core can be formed and attached to the deflector. For example, multiple layers of magnetic tape are wound around a surface of a cylindrical mold and bound together to form a laminate. The core is formed by cutting the laminate into a desired shape. The core is removed from the cylindrical mold and placed into a coil interior formed on the cylindrical substrate. The core can be attached using an adhesive and guided into position using multiple positioning features formed on the substrate surface.
An inner coil can be formed by a procedure as summarized above. The inner coil may be axially oriented ninety degrees apart from the outer coil. At least one through-hole can be formed on the cylindrical substrate, through which respective ends of the inner coil and the outer coil can be connected.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.